Sheet molded compound (SMC) parts are being increasingly investigated for use as replacements for conventional steel exterior automotive body panels such as rear deck lids, hoods, roof panels and, to some extent, doors. These exterior body panels are characterized by a generally flat large major surface. It has been difficult to provide these panels with smooth, pit free high quality finishes (referred to as Class A finishes in the industry) using conventional SMC compression molding techniques. Flaws or other surface deviations in the part after molding often require the use of filling and hand finishing operations to achieve the desired surface quality.
In recent years a process called "In-Mold Coating" has been delayed for the purpose of improving the surface quality of SMC parts. Basically, the in-mold coating process employs an additional operation whereby a coating material is injected onto the part while the molds are partially open. The molds are then reclosed and the coating material flows over the part surface filling pits, pores and cracks providing a nearly blemish-free coated surface.
Unfortunately, several problems have been encountered with this process. For example, the conventional use of ejector pins pressing against the underside of the major flat surface of the part to eject it from the mold often causes deformations that "telescope" or show through the upper coated surface thereby destroying its high quality finish. Another problem is that the part has a tendency to lift off of the lower male mold when the molds are opened to allow injection of the coating material. The resulting shifting or lifting of the part creates suction that may lodge debris underneath the part and cause further distortion when the molds are reclosed. If the part lifts a sufficient distance from the lower mold the coating material may actually be injected underneath the part instead of on its upper surface. In some instances this problem can also result in breaking or cracking the part when the molds are reclosed during curing of the coating material.